Passive emergency feedwater system

ABSTRACT

A power module assembly includes a reactor vessel containing a reactor core surrounded by a primary coolant. A containment vessel is adapted to be submerged in a containment cooling pool and to prohibit a release of the primary coolant outside of the containment vessel. A secondary cooling system is configured to remove heat generated by the reactor core. The heat is removed by circulating liquid from the containment cooling pool through the primary coolant.

This application claims priority to U.S. Provisional Application Ser.No. 60/988,348 filed on Nov. 15, 2007, the specification of which isherein incorporated by reference in its entirety.

TECHNICAL FIELD

The invention relates to a cooling system for a nuclear reactor.

BACKGROUND

In nuclear reactors designed with passive operating systems, the laws ofphysics are employed to ensure that safe operation of the nuclearreactor is maintained during normal operation or even in an emergencycondition without operator intervention or supervision, at least forsome predefined period of time. A Multi-Application Small Light WaterReactor project conducted with the assistance of the Idaho NationalEngineering and Environmental Laboratory, NEXANT and the NuclearEngineering Department of Oregon State University sought to develop asafe and economical natural light water reactor. FIG. 1 illustrates anuclear reactor design 5 that resulted from this project.

The nuclear reactor design 5 includes a reactor core 6 surrounded by areactor vessel 2. Water 10 in the reactor vessel 2 surrounds the reactorcore 6. The reactor core 6 is further located in a shroud 22 whichsurround the reactor core 6 about its sides. When the water 10 is heatedby the reactor core 6 as a result of fission events, the water 10 isdirected from the shroud 22 and out of a riser 24. This results infurther water 10 being drawn into and heated by the reactor core 6 whichdraws yet more water 10 into the shroud 22. The water 10 that emergesfrom the riser 24 is cooled down and directed towards the annulus 23 andthen returns to the bottom of the reactor vessel 2 through naturalcirculation. Pressurized steam 11 is produced in the reactor vessel 2 asthe water 10 is heated.

A heat exchanger 35 circulates feedwater and steam in a secondarycooling system 30 in order to generate electricity with a turbine 32 andgenerator 34. The feedwater passes through the heat exchanger 35 andbecomes super heated steam. The secondary cooling system 30 includes acondenser 36 and feedwater pump 38. The steam and feedwater in thesecondary cooling system 30 are isolated from the water 10 in thereactor vessel 2, such that they are not allowed to mix or come intodirect contact with each other.

The reactor vessel 2 is surrounded by a containment vessel 4. Thecontainment vessel 4 is placed in a pool of water 16. The pool of water16 and the containment vessel 4 are below ground 9 in a reactor bay 7.The containment vessel 4 is designed so that water or steam from thereactor vessel 2 is not allowed to escape into the pool of water 16 orthe surrounding environment. A steam valve 8 is provided to vent steam11 from the reactor vessel 2 into an upper half 14 of the containmentvessel 4. A submerged blowdown valve 18 is provided to release the water10 into suppression pool 12 containing sub-cooled water.

During a loss of feedwater flow, the nuclear reactor 5 is designed torespond by scramming the reactor core 6, flooding the containment vessel4 or depressurizing the reactor vessel 2. The latter two of theseresponses result in the nuclear reactor 5 being shut down and unable togenerate electricity for an extended period of time.

The present invention addresses these and other problems.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a nuclear power system known in the art.

FIG. 2 illustrates a novel power module assembly including an secondarycooling system.

FIG. 3 illustrates an embodiment of a novel emergency cooling system.

FIG. 4 illustrates a novel method of cooling a nuclear reactor.

SUMMARY OF THE INVENTION

A power module assembly is herein disclosed as comprising a reactorvessel containing a reactor core surrounded by a primary coolant. Acontainment vessel is adapted to be submerged in a containment coolingpool and to prohibit a release of the primary coolant outside of thecontainment vessel. A secondary cooling system is configured to removeheat generated by the reactor core, wherein the heat is removed bycirculating liquid from the containment cooling pool through the primarycoolant.

A cooling system for a nuclear reactor is herein disclosed as comprisingan inlet line configured to deliver coolant to a heat exchanger, and anoutlet line connected to the heat exchanger. The outlet line isconfigured to remove heat from the nuclear reactor. The cooling systemfurther comprises an inlet port adapted to supply emergency feedwater tothe inlet line, and an outlet port connected to the outlet line andconfigured to vent the emergency feedwater. The cooling system isconfigured to circulate the emergency feedwater through the heatexchanger by natural circulation.

A method of cooling a nuclear reactor is herein disclosed. The methodcomprises detecting a loss of feedwater condition and replacing afeedwater flow from a secondary cooling system with an emergencyfeedwater supply. The emergency feedwater is circulated through a heatexchanger to remove heat from the nuclear reactor. The emergencyfeedwater is circulated through natural circulation.

DESCRIPTION OF EXAMPLE EMBODIMENTS

Conventional nuclear facilities are expensive to license and build, withsignificant upfront investment costs and delayed return of profits. Inaddition to energy cost considerations, efficiency requirements, andreliability concerns, today's nuclear reactor designs must also takeinto account issues of nuclear proliferation, terrorist activities, anda heightened awareness of environmental stewardship.

Developing countries that could otherwise greatly benefit from nuclearpower are frequently left to resort to other energy sources such ascoal, gas or hydroelectric power generators that produce significantamounts of pollution or have other detrimental environmental impact.These developing countries may not have the technological or naturalresources that enable them to build a nuclear power plant. Countriesthat have already developed nuclear power may be hesitant to introducethese technologies into the developing countries out of concern of theloss of control of the nuclear materials or technology. Passively safenuclear power systems help address some of these concerns. Furthersystem improvements and innovative designs are expected to usher in anew era of nuclear power as a globally viable primary energy source.

In the Multi-Application Small Light Water Reactor (MASLWR) design, aloss of feedwater flow requires actuation of the long-term cooling modeof operation to provide for core cooling. For example, scramming thereactor core, flooding the containment vessel or depressurizing thereactor vessel. The latter two of these responses result in the nuclearreactor being shut down and unable to generate electricity for anextended period of time. Furthermore, no provision was provided in theMASLWR design for decay heat removal during a loss of site power.

FIG. 2 illustrates a novel power module assembly 25 including asecondary cooling system 50. The power module assembly 25 includes aninternally dry containment vessel 44. The containment vessel 44 iscylindrical in shape, and has spherical upper and lower ends. The entirepower module assembly 25 may be submerged in a containment cooling pool46 which serves as an ultimate heat sink. The containment vessel 44 maybe welded or otherwise sealed to the environment, such that liquids andgas do not escape from, or enter, the power module assembly 25. Thecontainment vessel 44 may be bottom supported, top supported orsupported about its center. Supporting the containment vessel 44 at thetop may facilitate maintenance and removal of the power module assembly25 from the containment cooling pool 46.

A reactor vessel 42 is located or mounted inside the containment vessel44. An inner surface of the reactor vessel 42 may be exposed to a wetenvironment including a primary coolant 100 or liquid, such as water,and an outer surface may be exposed to a dry environment such as air.The reactor vessel 42 may be made of stainless steel or carbon steel,may include cladding, and may be supported within the containment vessel44.

The power module assembly 25 may be sized so that it can be transportedon a rail car. For example, the containment vessel 44 may be constructedto be approximately 4.3 meters in diameter and 17.7 meters in height(length). By completely sealing the containment vessel 44, access to thereactor core 6 may be restricted. Any unauthorized access or tamperingmay be monitored. Furthermore, the subterranean profile of a nuclearpower system makes it less visible and easier to conceal. Thecontainment cooling pool 46 may be covered with a protective shield (notshown) to further isolate the power module assembly 25 from outsidethreats or airborne objects such as planes or missiles.

The containment vessel 44 encapsulates and, in some conditions, coolsthe reactor core 6. It is relatively small, has a high strength and maybe capable of withstanding six or seven times the pressure ofconventional containment designs in part due to its smaller overalldimensions. Given a break in the primary cooling system of the powermodule assembly 25 no fission products are released into theenvironment. The primary coolant 100 remains entirely contained in thecontainment vessel 44.

The reactor core 6 is illustrated as being submerged or immersed in aprimary coolant 100, such as water. The reactor vessel 42 houses theprimary coolant 100 and the reactor core 6. A shroud 22 surrounds thereactor core 6 about its sides and serves to direct the primary coolant100 up through an annulus 23 and out a riser 24 located in the upperhalf of the reactor vessel 42 as a result of natural circulation of theprimary coolant 100. In one embodiment, the reactor vessel 42 isapproximately 2.7 meters in diameter and includes an overall height(length) of 13.7 meters. The reactor vessel 42 may include apredominately cylindrical shape with spherical upper and lower ends. Thereactor vessel 42 is normally at operating pressure and temperature. Inone embodiment, the containment vessel 44 is internally dry and mayoperate at atmospheric pressure with wall temperatures at or near thetemperature of the containment cooling pool 46.

During normal operation, thermal energy from the fission events in thereactor core 6 causes the primary coolant 100 to heat. As the primarycoolant 100 heats up, it becomes less dense and tends to rise up throughthe riser 24. As the primary coolant 100 cools down, it becomesrelatively denser than the heated coolant and is circulated around theoutside of the annulus 23, down to the bottom of the reactor vessel 42and up through the shroud 22 to once again be heated by the reactor core6. This natural circulation causes the primary coolant 100 to cyclethrough the reactor core 6, transferring heat to a secondary coolingsystem 50 to generate electricity at a generator, such as generator 34of FIG. 1.

The secondary cooling system 50 comprises an inlet line 53 configured todeliver a secondary coolant to a heat exchanger 55 that operates as aheat sink for the power module assembly 25. An outlet line 52 isconnected to the heat exchanger 55 and is configured to remove heat fromthe reactor core 6 by circulating the secondary coolant through theprimary coolant contained in the reactor vessel 42. The inlet line 53and outlet line 52 therefore serve as a means of delivery of thesecondary coolant for the secondary cooling system 50. Relatively coolsecondary coolant is transported to the heat exchanger 55 via the inletline 53, whereas as relatively hot or superheated coolant is transportedfrom the heat exchanger 55 to the generator via the outlet line 52.

The secondary cooling system 50 further comprises one or more inletports 54 adapted to supply emergency feedwater to the inlet line 53. Inone embodiment, the emergency feedwater supply is contained in thecontainment cooling pool 46. The containment cooling pool 46 may containwater or some other liquid coolant. One or more outlet valves 58 may beconnected to the outlet line 52 and configured to vent the emergencyfeedwater after it has circulated through the heat exchanger 55. A checkvalve 56 may be provided between the inlet port 54 and the inlet line 53to restrict the flow of emergency feedwater or secondary coolant in onedirection. The inlet port 54 may include an inlet screen to filtercontaminants in the containment cooling pool 46.

The secondary cooling system 50 may be configured to circulate theemergency feedwater through the heat exchanger 55 by naturalcirculation. The natural circulation may be accomplished due to atemperature difference between the liquid in the containment coolingpool 46 and the primary coolant. The liquid, or emergency feedwater,undergoes a temperature change as it passes through the heat exchanger55. The natural circulation may further be accomplished or augmented asa result of the temperature change of the emergency feedwater and anelevation difference of the inlet ports 54 and the outlet valves 58. Inone embodiment, the one or more inlet ports 54 are located near thebottom of the containment cooling pool 46.

As previously described, the inlet line 53 may be connected to afeedwater pump, such as pump 38 of FIG. 1, and the outlet line 52 may beconnected to a steam turbine, such as turbine 32 of FIG. 1. Thesecondary cooling system 50 is able to provide natural circulation ofthe emergency feedwater without the assistance of a feedwater pump orexternal power source.

During a loss of feedwater flow scenario, a reactor scram and turbinetrip are initiated by an actuation signal. As steam pressure in thesecondary cooling system 50 increases, the one or more outlet valves 58are opened in a staged manner to depressurize the steam generator. Theone or more outlet valves 58 may include redundant fail safe vent valvesthat discharge the secondary coolant to a set of submerged spargernozzles located below the surface of the containment cooling pool 46.After a preset time delay from actuation of the outlet valves 58, asecond set of valves associated with the one or more inlet ports 54 maybe opened in such a way as to align the feedwater supply piping with thebottom of the containment cooling pool 46.

As the steam generator depressurizes, a natural circulation flowpath isestablished from the inlet ports 54 through the heat exchanger 55 andout the outlet valves 58. Liquid from the containment cooling pool 46provides a makeup source of emergency feedwater. Steam produced in theheat exchanger 55 and released through the outlet valves 58 is condensedin the containment cooling pool 46. The outlet valves 58 may bepositioned just below the surface of the containment cooling pool 46.

The secondary cooling system 50 may be configured to remove the liquidfrom the containment cooling pool 46 during one or more types ofemergency operation. The emergency operation may include a loss ofcoolant accident, a loss of secondary coolant flow, a loss of on-sitepower, among others.

FIG. 3 illustrates an embodiment of a novel cooling system 60. Thecooling system may be configured to operate with a nuclear reactor suchas the power module assembly 25 of FIG. 2. The cooling system 60comprises inlet line 53 and outlet line 52 configured to remove heatfrom a reactor core by circulating a coolant through heat exchanger 55.Relatively cool coolant is transported to the heat exchanger 55 via theinlet line 52, whereas as relatively hot or superheated coolant istransported from the heat exchanger 55 to the generator via the outletline 52.

The cooling system 60 further comprises one or more inlet ports 54adapted to supply emergency feedwater to the inlet line 53. In oneembodiment, the emergency feedwater supply is contained in a coolingpool 46 (FIG. 2), and comprises water or some other coolant. A coolingpipe 57 connecting the inlet port 54 to the inlet line 53 may beinsulated to reduce an amount of heating of the emergency feedwatertraveling through the cooling pipe 57 that might otherwise occur due toa temperature difference of the coolant located at the top and bottom ofthe cooling pool. One or more outlet pots 58 may be connected to theoutlet line 52 to vent the emergency feedwater after it has circulatedthrough the heat exchanger 55. The emergency cooling system 60 furthercomprises one or more accumulator tanks 70 configured to inject coolantinto the inlet line 53 when a loss of feedwater flow is detected. Theone or more accumulator tanks 70 provide coolant to the heat exchanger55 until natural circulation of the emergency feedwater is establishedvia the inlet ports 54 and outlet ports 58.

The one or more accumulator tanks 70 may be partially filled with water.The accumulator tanks 70 may be pressurized with a non-condensable gas,such as nitrogen. In one embodiment, a bladder 71 is provided with, orin, the accumulator tank 70 to prevent the release of thenon-condensable gas (e.g. nitrogen) into the heat exchanger 55. During aloss of feedwater flow scenario, the accumulator tanks 70 inject thewater into the inlet line 53. The injection of water serves to subcoolthe water in the reactor vessel while natural circulation is establishedin the cooling system.

Operation

Example operations of various embodiments is now provided, makingreference to the secondary cooling system 50 of FIG. 2 and the emergencycooling system 60 of FIG. 3. The reactor core 6 undergoes a hot shutdowncondition with control rods inserted. The shutdown condition may resultfrom an emergency operation of the power module assembly 25 or thesecondary cooling system. A normal steam flow through the outlet line 52and a feedwater flow through the inlet line 53 are isolated. The inletport 54 and outlet valve 58 are opened to the containment cooling pool46. This creates a natural circulation flow path driven by the coolantdensity difference and elevation difference between the cold water atthe inlet port 54 and the outlet valve 58.

Cold water from the containment cooling pool 46 is drawn into the heatexchanger 55 where it is heated and vented into the containment coolingpool 46. The heat exchanger 55 removes heat from fluid, for example thecoolant 100, in the annulus 23 creating a density difference between thefluid inside the riser 24 and the fluid in the annulus 23. Because thereactor core 6 is located at an elevation below the heat exchanger 55, abuoyancy force is created that drives warm fluid up through the shroud22 and riser 24 and drives cold fluid down through the annulus 23 intothe lower plenum 51. This creates a natural circulation flow through thereactor core 6 that removes the decay heat. The check valve 56 limitsfluid flow in one direction.

During a loss of main feedwater flow, a low discharge pressure of theLow Main Feedwater Pump 72 or a low water level of the Steam Generator74 may result in a reactor trip. After a time delay from the reactortrip, the main steam vent valves (outlet valves 58) are opened in astaged manner. When a low level of the Accumulator tank 70 and a lowdischarge pressure of the Low Main Feedwater Pump 72 are detected, theMain Feedwater Stop Valve 76 is closed. Then, the inlet valve (inletports 54) is opened in order to align the cooling flow to thecontainment cooling pool 46. A check valve 66 prevents reverse flow fromthe Steam Generator 74 and a high point vent 68 provides for the initialpurging of air in the emergency feedwater line.

In various embodiments, the same operations as discussed for the aboveloss of main feedwater flow condition may be followed during a stationblackout or loss of power to the on-site facility.

During a loss of coolant accident, a low water level of the LowPressurizer 73, a low pressure of the coolant system, or a high pressureof the containment vessel 44 may result in a reactor trip. The MainSteam Isolation Valve 78 and Main Feedwater Stop Valve 76 are closed. Ifthe steam generator pressure 77 does not increase (e.g. there is nosteam generator tube rupture) and the steam generator tube bandpressures are equal, then the outlet valves 58 may be opened in a stagedmanner. For example, the Main Feedwater Stop Valve 76 is closed when alow level of the Accumulator tank 70 and a low discharge pressure of theLow Main Feedwater Pump 72 are detected. Additionally, the inlet port 54may be opened to align the cooling flow to the containment cooling pool46. The outlet valves 58 may also be opened in a staged manner. Areactor sump valve may be opened when a low differential pressure isdetected between the containment vessel 44 and the reactor vessel 42.

FIG. 4 illustrates a novel method of cooling a nuclear reactor. Atoperation 410, a loss of feedwater condition is detected. The loss offeedwater may be due to a loss of coolant accident, a loss of feedwaterpressure, a failed feedwater pump, or a loss of on-site power, such as astation blackout. At operation 420 a feedwater flow from a secondarycooling system is replaced with an emergency feedwater supply.

At operation 430 the emergency feedwater is circulated through a heatexchanger to remove heat from the nuclear reactor. The emergencyfeedwater is circulated through the heat exchanger through naturalcirculation. The natural circulation is due to a difference intemperature between the emergency feedwater supply and the emergencyfeedwater circulating through the heat exchanger.

In one embodiment, the emergency feedwater supply comprises acontainment cooling pool surrounding the nuclear reactor. At operation440, the emergency feedwater is vented into the containment coolingpool. An elevation difference between an outlet port and an inlet portsubmerged in the containment cooling pool may provide for sustainablenatural circulation of the emergency feedwater for more than three days.Depending on the size of the containment cooling pool, in oneembodiment, the natural circulation may be maintained upwards of 90days.

Loss of feedwater flow and decay heat removal is resolved by the variousembodiments disclosed herein. The novel systems add significantcapability to the MASLWR reactor design by providing a passive means ofcooling the nuclear core after a control rod insertion without the needfor external power. Various embodiments are able to provide emergencyfeedwater to the steam generator in the event of a loss of normalfeedwater flow, and provide reactor core decay heat removal of thereactor core subsequent to a reactor control rod insertion.

The containment cooling pool serves as a source of makeup feedwater andas a heat sink for decay heat removal. Various embodiments are able toremove core decay heat by directing coolant from the large containmentcooling pool through the helical coil heat exchanger tubes located inthe reactor vessel annulus. A natural circulation flow path isestablished as hot water and steam are vented into the containment pooland cold water is drawn into the inlet port.

Various embodiments disclosed herein provide alternate long-term coolingmode of operation and an indefinite heat removal of the reactor corewithout operator action. Very little mass is lost form the containmentpool. Various embodiments can be actuated manually to remove decay heatfor maintenance, and the systems are relatively simple, having few ifany moving parts. Passive systems do not require on-site power tooperate, instead relying on the principles of natural circulation.Furthermore, various embodiments provide for quick restart of a reactormodule, providing less operating downtime and increased protection ofinvestor capital.

Although the embodiments provided herein have primarily described apressurized water reactor, it should be apparent to one skilled in theart that the embodiments may be applied to other types of nuclear powersystems as described or with some obvious modification. For example, theembodiments or variations thereof may also be made operable with aboiling water reactor. A boiling water reactor may require largervessels to produce the same energy output.

Having described and illustrated the principles of the invention in apreferred embodiment thereof, it should be apparent that the inventionmay be modified in arrangement and detail without departing from suchprinciples. We claim all modifications and variation coming within thespirit and scope of the following claims.

1. A power module assembly comprising: a primary cooling circuitcomprising a reactor vessel and a reactor core surrounded by a primarycoolant, the primary cooling circuit configured to promote a naturalcirculation flowpath within the reactor vessel; a containment vesselsubmerged in a containment cooling pool and configured to prohibitrelease of the primary coolant outside of the containment vessel; a heatexchanger disposed within the containment vessel in fluid communicationwith the primary coolant, the heat exchanger configured to remove heatfrom the primary coolant; a secondary cooling circuit including a steamgeneration circuit configured to generate steam, using heat extractedfrom the primary coolant, to drive a turbine; an emergency coolingsystem, isolated from the steam generation circuit, and configured to becoupled to the heat exchanger during emergency operation to remove heatfrom the primary coolant via natural circulation of an emergencycoolant, without a pump or external power source.
 2. The power moduleassembly of claim 1, wherein the steam generation circuit furthercomprises an accumulator tank.
 3. The power module assembly of claim 1,wherein the emergency cooling system comprises at least one valve thatisolates the emergency cooling system from the steam generation circuitduring emergency operation.
 4. The power module assembly of claim 1,wherein the emergency coolant circulates through the heat exchanger bythe natural circulation induced by a temperature difference between theemergency coolant and the primary coolant.
 5. The power module assemblyof claim 1, wherein heat removed from the primary coolant is depositedin the containment cooling pool.
 6. The power module assembly of claim 1wherein the emergency coolant comprises coolant drawn from thecontainment cooling pool.
 7. The power module assembly of claim 1wherein the emergency coolant enters the emergency cooling circuit viaan inlet disposed in the containment cooling pool.
 8. The power moduleassembly of claim 7 wherein the emergency coolant exits the emergencycooling circuit via an outlet disposed in the containment cooling pool.9. The power module assembly of claim 8, wherein the outlet ispositioned at an elevation of the containment cooling pool that is abovean elevation of the inlet, and wherein the emergency coolant circulatesthrough the primary coolant by the natural circulation at least partlydue to an elevation difference between the inlet and the outlet.
 10. Thepower module assembly of claim 1, wherein the emergency cooling circuitfurther comprises an inlet line configured to deliver the emergencycoolant from an inlet to the heat exchanger and an outlet lineconnecting the heat exchanger to an outlet.
 11. The power moduleassembly of claim 1, wherein the containment vessel is internally drybefore emergency operation.
 12. A power module assembly comprising: areactor vessel containing a reactor core surrounded by a primarycoolant, internal structures within the reactor vessel configured topromote natural circulation of the primary coolant; containment meansconfigured to prohibit a release of the primary coolant into asurrounding containment cooling pool; a heat exchanger disposed in fluidcommunication with the primary coolant to remove heat generated by thereactor core; a steam generation circuit configured to generate steam,using heat extracted from the primary coolant, to drive a turbine; andan emergency cooling circuit, isolated from the steam generationcircuit, and configured to be coupled to the heat exchanger duringemergency operation to circulate an emergency coolant to remove heatfrom the primary coolant, wherein the emergency coolant is circulatedthrough the heat exchanger and the emergency cooling circuit via naturalcirculation.
 13. The power module assembly of claim 12, wherein theemergency cooling circuit comprises an inlet submerged in thecontainment cooling pool.
 14. The power module assembly of claim 13,wherein the emergency cooling circuit further comprises an outletpositioned at an elevation in the containment cooling pool that ishigher than an elevation of the inlet.
 15. The power module assembly ofclaim 13, wherein natural circulation is established in the emergencycooling system at least partly due to a difference in density of theemergency coolant at the inlet and the emergency coolant at the outlet.16. The power module assembly of claim 12, wherein the containmentvessel is internally dry before emergency operation.
 17. The powermodule assembly of claim 12, wherein the emergency cooling circuitfurther comprises at least one valve that isolates the emergency coolingsystem from the steam generation circuit during emergency operation. 18.The power module assembly of claim 12, wherein heat removed from theprimary coolant is deposited in the containment cooling pool.
 19. Thepower module assembly of claim 12, wherein the steam generation circuitfurther comprises an accumulator tank.
 20. The power module assembly ofclaim 12, wherein the emergency cooling circuit further comprises aninlet line configured to deliver the emergency coolant from an inlet tothe heat exchanger and an outlet line connecting the heat exchanger toan outlet.